将变量从 Scala 中存在类型的 TypeCast 转换为运行时类型

Question

This thread addressed how to use TypeTag to get runtime type of type parameters when used with Existential type. Another thread addressed how to cast a variable to its runtime type retrieved from TypeTag .

My question builds on the aforementioned threads (kind of a combination of the two scenarios). Parts of the code is duplicated from the two threads for clarity.

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> def cast[A](a: Any, tt: TypeTag[A]): A = a.asInstanceOf[A]
cast: [A](a: Any, tt: reflect.runtime.universe.TypeTag[A])A

scala> abstract class Animal[T](implicit tt: TypeTag[T]) {
     |   val ttag = tt
     | }
defined class Animal

scala> case object Dog extends Animal[Int]
defined object Dog

scala> case object Cat extends Animal[String]
defined object Cat

scala> val aa: List[(Animal[_], Any)] = List((Dog, 5), (Cat, "stringgg"), (Dog, 2))                                                                                                               
aa: List[(Animal[_], Any)] = List((Dog,5), (Cat,stringgg), (Dog,2))

scala> aa(0)._1.ttag
res25: reflect.runtime.universe.TypeTag[_] = TypeTag[Int]

scala> aa(1)._1.ttag
res26: reflect.runtime.universe.TypeTag[_] = TypeTag[String]

scala> cast(aa(0)._2, aa(0)._1.ttag)
res27: Any = 5

scala> cast(aa(1)._2, aa(1)._1.ttag)
res28: Any = stringgg

In short, the last two lines: cast(value, TypeTag[_]) always return a value of type Any . But my intention is to cast these values to the correct type stored in Dog|Cat.ttag , which, unfortunately, is not TypeTag[Int] or TypeTag[String] but TypeTag[_] due to usage of existential type. Is there any solution for it?

Edit 1:

Despite the field ttag is of type TypeTag[_] , ttag.tpe does have the right "type" (but as an instance of reflect.runtime.universe.Type ). Is it possible to use the typetag.tpe to cast to the right type?

Answer 1

你可以改变cast方法， def cast[A](a: Any, tt: TypeTag[_]): A = a.asInstanceOf[A]和

val x: Int = cast(aa(0)._2, aa(0)._1.ttag)

Answer 2

pair with path dependent type, you can type cast as following code shows:

import languageFeature.existentials

object TestExistential {
  def main(args: Array[String]): Unit = {
    val aa: Array[(Animal[T], T) forSome {type T}] = Array((Dog, 5:Integer), (Cat, "stringgg"), (Dog, 2:Integer))
    aa.foreach(p=>{
      val ani = p._1
      val attr = p._2 // compiler weakness: existential type T is not unify with ani.castType
      assert(ani.attrTyp.isAssignableFrom(attr.getClass))// because of boxing, real type is Integer not Int
      assert(ani.clzTyp.isAssignableFrom(ani.getClass))
      val typedAni:ani.castType = ani.asInstanceOf[ani.castType]
      println(s"$typedAni of type Animal[${ani.attrTyp}] with $attr")
    })
  }

  abstract class Animal[T]{
    type castType = Animal[T]
    val attrTyp: Class[T]
    val clzTyp: Class[castType]
  }

  case object Dog extends Animal[Integer] {
    override val attrTyp: Class[Integer] = classOf[Integer]
    override val clzTyp: Class[Dog.castType] = classOf[Dog.castType]
  }

  case object Cat extends Animal[String] {
    override val attrTyp: Class[String] = classOf[String]
    override val clzTyp: Class[Cat.castType] = classOf[Cat.castType]
  }
}

if typing in scala repl, you can see the reply:

scala> val typedAni:ani.castType = ani.asInstanceOf[ani.castType]

typedAni: ani.castType = Dog

scala> typedAni

res0: ani.castType = Dog